SRP (stands for Software Rendering Pipeline) is an OpenGL-like CPU graphics library written in C with minimal dependencies. It was made as a hobby programming project, so it is probably not the best idea to use it in production code (see PortableGL, for example). The source code (and more information) is available on Github.

A nice way to get familiar with API is to read Examples or API reference. If you are interested in implmentation details, see Implementation.

Here is a graph showing the pipeline (red edges - the main path from a draw call to a fragment on the screen, green nodes - objects created by user):